Most automobiles sold today incorporate an inflatable occupant restraint system (Supplemental Restraint System or SRS airbags). These systems are designed to be activated in the event of a frontal collision to protect the driver, and sometimes the front passenger, from injury. In a collision, the system is activated to inflate an airbag which restrains the occupants and protects them from the forces generated during the collision. In most instances, the method of deploying the bag includes electrical ignition of a gas-generating chemical, such as sodium azide, that quickly inflates the bag during the frontal impact.
Federal standards require that the inflating system used in automobile airbag restraint systems be designed to function during exposure to fire while maintaining inflator body integrity. This requirement is intended to protect nearby persons and property should the system be exposed to a fire while in the car, or when the device is uninstalled and being shipped. However, the casing containing the inflating mechanism may become weakened when exposed to the high heat of a fire. When the inflating system is activated under these conditions, the weakened casing may rupture and propulsively propel sharp pieces of the casing and very hot gases and particles. This situation creates the potential for injuring nearby persons, such as firefighters, or damage to nearby property.
One solution to overcome the problem of a weakened casing during exposure to fire is to formulate an activating propellant that ignites at a lower temperature than the temperature at which the casing is weakened. This solution integrates a special autoignition propellant which autoignites at a temperature that is lower than that of the main propellant. Once ignited, the autoigniting propellant lights the main propellant which inflates the airbag.
Several disadvantages occur with many typical autoignition propellants. First, many of the autoignition propellants autoignite at temperatures around 200.degree. C. As the main propellant approaches this temperature (especially likely at low heating rates), the burning characteristics change to make combustion more rapid. This increased rate of propellant combustion increases the probability of deformation or damage to the inflator casing following ignition.
Another disadvantage of many autoignition propellants that autoignite at approximately 200.degree. C. is the time required to transfer heat to the inside of the inflator where the autoignition propellant is located. The relatively long exposure time required to heat the autoignition propellant to its autoignition temperature causes the exterior of the inflator casing to become hotter than the required autoignition temperature. At these elevated temperatures, the casing has lower tensile strength which increases the probability of deformation during autoignition.
In some instances, additional devices are integrated into the inflation mechanism to inflate the restraint during a fire. However, some inflator components, for example initiators, are fabricated with materials that may deform or degrade at the high temperatures encountered during a fire. The deformed components may allow gases to leak and propel the activating unit into the passenger compartment following autoignition.
U.S. Pat. No. 5,380,380 to Poole et al. discloses an automobile occupant restraint system that will autoignite and cause ignition of a gas generant when heated to approximately 150.degree.-210.degree. C., thereby permitting use of an aluminum pressure vessel. The autoigniting composition contains a hydrazine salt of 3-nitro-1,2,4-triazole-5-one, boron, and potassium nitrate.
U.S. Pat. No. 5,084,118 to Poole discloses an autoigniting composition for the gas generator of a vehicle occupant restraint system that autoignites when heated to approximately 150.degree.-210.degree. C. The autoignition composition is made from sodium chlorate, 5-aminotetrazole, and 2,4-dinitrophenylhydrazine.
Published PCT application WO 95/04710 discloses a gas generant composed of phase-stabilized ammonium nitrate, a nitrogen containing fuel, and an optional organic polymer binder.